Inkjet printing is a popular and versatile form of print imaging. The Assignee has developed printers that eject ink through MEMS printhead IC's. These printhead IC's (integrated circuits) are formed using lithographic etching and deposition techniques used for semiconductor fabrication.
The micro-scale nozzle structures in MEMS printhead IC's allow a high nozzle density (nozzles per unit of IC surface area), high print resolutions, low power consumption, self cooling operation and therefore high print speeds. Such printheads are described in detail in U.S. Ser. No. 10/160,273 filed Jun. 4, 2002 and U.S. Ser. No. 10/728,804 filed on Dec. 8, 2003 to the present Assignee. The disclosures of these documents are incorporated herein by reference.
The small nozzle structures and high nozzle densities can create difficulties with color mixing between nozzles of different color. During periods of prolonged inactivity (or ‘standby mode’) the separate fluidic lines for each ink color can undergo slight pressure changes relative to each other. Different rates of heating and outgassing in different ink lines will generate a slight pressure differential. If paper dust or ink residue on the nozzle face extends between nozzles of the different ink lines, the dust or residual ink can forge a fluid connection between the two ink lines. The ink lines try to equalize the pressure difference between them and this drives an ink from the higher pressure line to the lower pressure line. If left unchecked, the ink contamination in the lower pressure ink line can extend to the ink tank. In this case, the contaminated ink supply is irretrievable and needs replacement before the ink lines are flushed through to the nozzles.
The ink tanks can be isolated from the printhead by a shut off valve upstream of the printhead. This protects the tanks from contamination during standby, but there is a risk that the tank and the printhead will generate a pressure difference during the standby period. If this happens, the sudden pressure equalization causes a pulse through the ink line which floods the nozzle plate.